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Pension and other Post-Retirement Benefits - Investment Objectives (Details) - Defined benefits pension plans - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension and other PostRetirement Benefits,    
Employer contributions $ 160  
Contribution deposited in solvency reserve account 3  
Expected cash contributions $ 174  
Equities    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 56.00% 59.00%
Equities | Canada    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 18.00% 19.00%
Equities | United States    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 19.00% 23.00%
Equities | Foreign    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 19.00% 17.00%
Fixed income | Canada    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 39.00% 39.00%
Real estate    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 100.00% 100.00%
Real estate | Canada    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 5.00% 2.00%
Maximum | Fixed income    
Pension and other PostRetirement Benefits,    
Plan asset allocation percentage 44.00%